A widely used technique for searching for oil or gas is the seismic exploration of subsurface geophysical structures. The seismic exploration process consists of generating seismic waves (i.e., sound waves) directed toward the subsurface area, gathering data on reflections of the generated seismic waves at interfaces between layers of the subsurface, and analyzing the data to generate a profile (image) of the geophysical structure, i.e., the layers of the investigated subsurface. This type of seismic exploration can be used both on the subsurface of land areas and for exploring the subsurface of the ocean floor.
It is known by those of ordinary skill in the art of seismic exploration that an appropriate choice of frequencies to drive a sound producing device can be used to generate seismic waves whose reflections can, in turn, be used to determine the possible or probable location of hydrocarbon deposits under, e.g., the ocean floor. The sound producing device in such marine applications can be referred to as a marine vibrator, and is generally also called a “source,” i.e., a source of the sound waves that are transmitted and then reflected/refracted off the ocean floor and then received by one or more, usually dozens, of receivers. Marine vibrators (herein after referred to as “vibrators,” “marine vibrators,” and/or “seismic vibrators”) can be implemented in what are referred to as “towed arrays” of the plurality of sources and their receivers, wherein each towed array can include numerous vibrators, numerous receivers, and can include several or more groups of receivers, each on its own cables, with a corresponding source, again on its own cable. Systems and methods for their use have been produced for devices that can maintain these cables, for example, in relatively straight lines as they are being towed behind ships in the ocean. As those of ordinary skill in the art can appreciate, an entire industry has been created to explore the oceans for new deposits of hydrocarbons, and has been referred to as “reflection seismology.”
For a seismic gathering process, as shown in FIG. 1, a data acquisition system 10 includes a ship 2 towing plural streamers 6 that may extend over kilometers behind ship 2. Each of the streamers 6 can include one or more birds 13 that maintains streamer 6 in a known fixed position relative to other streamers 6, and the birds 13 are capable of moving streamer 6 as desired according to bi-directional communications birds 13 can receive from ship 2. One or more source arrays 4a,b may be also towed by ship 2 or another ship for generating seismic waves. Source arrays 4a,b can be placed either in front of or behind receivers 14, or both behind and in front of receivers 14. The seismic waves generated by source arrays 4a,b propagate downward, reflect off of, and penetrate the seafloor, wherein the refracted waves eventually are reflected by one or more reflecting structures (not shown in FIG. 1) back to the surface (see FIG. 2, discussed below). The reflected seismic waves propagate upwardly and are detected by receivers 14 provided on streamers 6. This process is generally referred to as “shooting” a particular seafloor area, and the seafloor area can be referred to as a “cell”.
FIG. 2 illustrates a side view of the data acquisition system 10 of FIG. 1. Ship 2, located on ocean surface 46, tows one or more streamers 6, that is comprised of cables 12, and a plurality of receivers 14. Shown in FIG. 2 are two source streamers, which include sources 4a,b attached to respective cables 12a,b. Each source 4a,b is capable of transmitting a respective sound wave, or transmitted signal 20a,b. For the sake of simplifying the drawings, but while not detracting at all from an understanding of the principles involved, only a first transmitted signal 20a will be shown (even though some or all of source 4 can be simultaneously (or not) transmitting similar transmitted signals 20). First transmitted signal 20a travels through ocean 40 and arrives at first refraction/reflection point 22a. First reflected signal 24a from first transmitted signal 20a travels upward from ocean floor 42, back to receivers 14. As those of skill in the art can appreciate, whenever a signal—optical or acoustical—travels from one medium with a first index of refraction n1 and meets with a different medium, with a second index of refraction n2, a portion of the transmitted signal is reflected at an angle equal to the incident angle (according to the well-known Snell's law), and a second portion of the transmitted signal can be refracted (again according to Snell's law).
Thus, as shown in FIG. 2, first transmitted signal 20a generates first reflected signal 24a, and first refracted signal 26a. First refracted signal 26a travels through sediment layer 16 (which can be generically referred to as first subsurface layer 16) beneath ocean floor 42, and can now be considered to be a “new” transmitted signal, such that when it encounters a second medium at second refraction/reflection point 28a, a second set of refracted and reflected signals 32a and 30a, are subsequently generated. Further, as shown in FIG. 2, there happens to be a significant hydrocarbon deposit 44 within a third medium, or solid earth/rock layer 18 (which can be generically referred to as second subsurface layer 18). Consequently, refracted and reflected signals are generated by the hydrocarbon deposit, and it is the purpose of data acquisition system 10 to generate data that can be used to discover such hydrocarbon deposits 44.
The signals recorded by seismic receivers 14 vary in time, having energy peaks that may correspond to reflectors between layers. In reality, since the sea floor and the air/water are highly reflective, some of the peaks correspond to multiple reflections or spurious reflections that should be eliminated before the geophysical structure can be correctly imaged. Primary waves suffer only one reflection from an interface between layers of the subsurface (e.g., first reflected signal 24a). Waves other than primary waves are known as multiples. Signal 50a shown in FIG. 2 is one such example of a multiple, but as shown in FIG. 3, there are other ways for multiples to be generated.
As illustrated in FIG. 3, seismic source 4 produces first transmitted wave 20a that splits into a primary transmitted wave 26a (referred to also as first refracted signal) penetrating inside first subsurface layer 16 (referred to also as “sediment layer” though that does not necessarily need to be the case) under ocean floor 42, and surface related multiple signal 50 that travels back towards ocean surface 46 (or fourth interface). Primary transmitted wave 26a is reflected once at second interface 48 between different layers in first subsurface layer 16 and travels back to receiver 14 as second reflected signal 30a. Surface related multiple signal 50 also reaches receiver 14, but at a different time, after being reflected (at least) two more times: a first reflection at surface 46 and a second reflection at sea floor 42. Thus, receiver 14 will receive at least two different signals from the same transmitting event: second reflected signal 30a, and surface related multiple signal 50. Surface related multiple signal 50 can be received by receiver 14 either before second reflected signal 30a, at the same time, or after, depending on how far first refracted signal 26a traveled in first subsurface layer 16, and how deep ocean 40 is at the point of transmission and reflection/refraction. Thus, receiver 14 can become “confused” as to the true nature of the subsurface environment due to surface multiple signal(s) 50. As briefly discussed above, other multiples can also be generated, some of which may also travel through the subsurface. A multiple, therefore, is any signal that is not a primary reflected signal. Further, a different type of multiple signal is also shown in FIG. 3, internal multiple signal 51, which experiences a downward reflection from an underground layer, such as shown in FIG. 3. “Multiples”, as is known by those of ordinary skill in the art, can cause problems with determining the true nature of the geology of the earth below the ocean floor. Multiples (whether surface related multiples 50 or internal multiples 51) can be confused by data acquisition system 10 with first, second or third reflected signals. Multiples do not add any useful information about the geology beneath the ocean floor, and thus they are, in essence, noise, and it is desirable to eliminate them and/or substantially reduce and/or eliminate their influence in signal processing of the other reflected signals so as to correctly ascertain the presence (or the absence) of underground/underwater hydrocarbon deposits.
While surface multiples 50 cause some problems with signal processing to determine the “true” nature of the underwater subsurface geology, internal multiples 51 have been known to be especially problematic. Internal multiples 51 typically arise due to a series of subsurface impedance contrasts. They are commonly observed in seismic data acquired in various places, such as the Santos Basin of Brazil. They are often poorly discriminated from the primaries (i.e., the first, second and third reflected signals, among others), because they have similar movement, dips and frequency bandwidth, thereby making attenuation and/or elimination of internal multiples 51 one of the key issues in providing clear seismic images in interpreting areas of interest. Over time, various methods have been developed to address this difficult problem and most of them rely on the ability to identify the multiple generators. Approaches, such as Delft's feedback model (Verschuur, D. J. et al., 1996, “Removal of Inter-bed Multiples,” 58th Meeting, EAGE, Expanded Abstracts, Paper B003, the entire contents of which are incorporated herein by reference), Jakubowicz' convolution-correlation method (Pica, A. et al., 2008, “Wave Equation Based Internal Multiple Modelling in 3D,” 78th Meeting, SEG, Expanded Abstracts, p. 2476-2480, the entire contents of which are incorporated herein by reference), model driven methods, and predictive de-convolution (Hugonnet, P. et al., 2005, “2D Deconvolution for OBC Data and for Internal Multiple Attenuation—Part 1: Theory,” 67th Meeting, EAGE, Extended Abstracts, Paper A026, the entire contents of which are incorporated herein by reference) require a priori information about the subsurface. When the information is available, perhaps via well logs or interpretation results, significant suppression of internal multiples can be observed in these methods. Nevertheless, in many situations, it is often not easy to identify the multiple generators and this makes the problem challenging.
Other methods for handling multiples have also been developed that do not require a priori subsurface reflector information. For example, a methodology has been developed using inverse scattering series (ISS) (Otnes, E. et al., 2004, in “Attenuation of Internal Multiples for Multicomponent and Towed Streamer Data,” 74th Meeting, SEG, Extended Abstracts, p. 1297-1300, the entire contents of which is incorporated herein by reference), has been applied on marine and land data for internal multiple attenuation. The ISS method is a data-driven approach that can predict all internal multiples of a given order without any subsurface information. As those of skill in the art can appreciate, order of multiples refers to the number of downward bounces a wavefield experiences prior to being captured by a receiver. In comparison, Delft's feedback model or Jakubowicz' method removes all orders of internal multiples for a given interface (see, Verschuur, D. J. et al., 1999, “A Comparison of the Feedback and Inverse Scattering Internal Multiple Attenuation Methods,” 61st Meeting, EAGE, Extended Abstracts, p. 1-14, the entire contents of which are incorporated herein by reference). There are significant differences between the two classes of modelling methodology. One difference is that internal multiples are catalogued differently and the other significant difference is the requirement for a priori information in the former class of modelling technologies (see, Matson, K. H. et al., 1998, “Comparing the Interface and Point Scatterer Methods for Attenuating Internal Multiples: A Study with Synthetic Data—Part II,” 68th Meeting, SEG, Extended Abstracts, p. 1523-1526, the entire contents of which are incorporated herein by reference). However, it has been asserted that, for surface-related multiple attenuation, the two methods are the same in theory (see, Levin, S. A., 2008, “Delft Inverse Scattering Surface-Related Multiple Attenuation in Three Lines,” 78th Meeting, SEG, Extended Abstracts, p. 2512-2515, the entire contents of which are incorporated herein by reference).
One of the important requirements in ISS that allows it to predict internal multiples without subsurface information is the requirement for a pseudo-depth monotonicity condition (see, Nita, B. G. et al., 2007, “Inverse Scattering Internal Multiple Attenuation Algorithm: An Analysis of the Pseudo-Depth and Time Monotonicity Requirements,” 77th Meeting, SEG, Expanded Abstracts, p. 2461-2464, the entire contents of which are incorporated herein by reference). The pseudo-depth monotonicity condition basically means that, for a particular internal multiple event, the point scatterer that causes the downward reflection is at a higher depth (in pseudo-depth) than that of the point scatterers that cause the upward reflections, i.e., satisfying a “lower-higher-lower” relationship.
The inherent limitation in the ISS approach of pseudo-depth monotonicity limits its ability to be widely used. Accordingly, it would be desirable to provide methods, modes and systems for effectively and efficiently eliminating the influence of internal multiples when determining sub-ocean floor geology, in order to make it easier to determine the presence (or not) of sub-surface hydrocarbons.